Magenta DAC 2.1 User Manual - page 3
Resolution
A lot of people think USB audio is limited to 16 bits/48 kHz max.
A lot of (cheap and sometimes not so cheap) USB DACs are indeed limited
to this resolution.
Another common misunderstanding is the specification of the bus (USB 1,2
or 3) and the USB audio standard (1 or 2).
USB Audio Class 1 standard (1998)
This standard allows for 24 bits/96 kHz max.
The standard itself doesn't impose any limitation on sample rate.
Class 1 is tied to USB 1 Full Speed = 12 MHz
Every millisecond a package is send.
Maximum package size is 1024 bytes.
2 channel * 24 bit * 96000 Hz sample rate= 4608000 bits/s or 576 Byte/ms
This fits in the 1024 byte limit.
Any higher popular sample rate e.g. 176 kHz needs 1056 bytes so in excess
of the maximum package size.
All operating systems (Win, OSX, and Linux) support USB Audio Class 1
natively.
This means you don’t need to install drivers, it is plug&play.
All support 2 channel audio with 24 bit words and 96 kHz sample rate
USB Audio Class 2 standard (2009)
It is downwards compatible with class 1.
USB Audio Class 2 additionally supports 32 bit and all common sample
rates > 96 kHz
Class 2 uses High Speed (480 MHz). This requires USB 2 or 3.
As the data rate of High Speed is 40 X Full speed, recording a 60 channel
using 24 bits at 96 kHz (132 Mbit/s) is not a problem.
From mid-2010 on USB audio class 2 drivers are available in OSX 10.6.4 and
Linux.
Both support sample rates up to 384 kHz.
It is unclear if Microsoft is going to support USB Audio 2.
You need a third party USB class 2 driver on Windows.
Using High Speed USB for playback there are no limits in resolution.
USB Speed
Superspeed - 10 Gbps USB data rate (USB 3.1)
Superspeed - 5 Gbps USB data rate (USB 3.0)
High Speed - 480 Mb/s with a data signalling tolerance of ± 500ppm
(USB 2).
This means every 125 µs a SOF packet arrives with a allowed deviation of
± 0.0625 µs..
Full Speed - 12 Mb/s with a data signalling tolerance of ±0.25% or
2,500ppm. (USB 1&2)
This means every 1ms a SOF packet arrives with a allowed deviation of ±
500ns.
Low Speed - 1.5Mbits/s with a data signalling tolerance of ±1.5% or
15,000ppm (USB 1&2)
USB cables
Cable length between full speed devices is limited to 5 meters. For
a low speed device the limit is 3 meters.
As the signal degrades proportional to the length of the cable, a
short cable is often recommended.
Other says this can put a source of RFI (the PC) to close to the
USB-DAC.
Audiophile USB cables
As file based audio is gaining momentum and many believe
asynchronous USB the way to go there is a growing market for
audiophile grade USB cables.
The question of course is why a cable can have any impact on
sound quality.